The present invention relates to a circuit for exhausting air from a parison, a system for supplying and exhausting air from a parison and an aseptic moulding process using said circuit and said system. In particular, the invention has application in the field of moulding containers made of plastic material, such as PET bottles, under aseptic conditions.
For the sake of clarity, the stretch-blowing process for moulding containers made of plastic material starting from heated parisons is briefly summarised below.
The heated parisons are made to advance towards a rotating carousel having a plurality of moulding stations. In each station there is present a mould consisting of two half-moulds which reproduce the shape of the sides of the container to be obtained. At one of the bases of the mould there is an element for shaping the bottom of the container, known in the art as “mould bottom”, which engages with the half-moulds to define a moulding cavity. Once the parison has been laterally clamped, the half-moulds are locked in a closed position by mechanical systems. Present at every mould there is also a closure element, called “seal”, having the task of hermetically sealing the mouth of the parison on the neck thereof and of delivering pressurised air into the parison itself.
In the initial phase of the moulding process, the seal delivers air into the parison at medium pressure (maximum of about 15 bar). While air is being delivered at medium pressure, a stretching rod is progressively inserted into the parison, until reaching the bottom of the same. After touching the bottom, the stretching rod continues its linear stroke in order to stretch the parison until the latter reaches approximately the length of the container to be obtained.
Subsequently, the seal blows air into the parison at high pressure (maximum of about 40 bar) so as to expand it until it adheres to the inner walls of the half-moulds and of the mould bottom. Simultaneously, the stretching rod retracts until it is outside the container. The air inside the container is maintained at a high pressure for a few moments so as to consolidate the moulding thereof.
The air inside the container is subsequently discharged, again through the seal. At the end of the process of emptying out the air, the seal is moved away from the container. The locking systems disengage from the two half-moulds, which can then be opened, enabling removal of the mould bottom and subsequent extraction of the moulded container.
An important part of the moulding apparatus is the circuit supplying air to be blown into the parison, which circuit is described below.
Situated downstream of the compressor which supplies air at a high pressure (up to about 40 bar) there are filtering means comprising at least a coalescing filter for eliminating aerosols of water and oil and solid particles, and an activated carbon filter for eliminating odours and oil and hydrocarbon vapours. Downstream of the filters, the supply line forks into a medium pressure circuit (maximum of about 15 bar) and a high pressure circuit (maximum of about 40 bar). Aseptic technology further requires the use of HEPA (acronym of “High Efficiency Particulate Air”) filters to remove particles of a size exceeding 0.2 μm from the air.
The air at medium pressure and the air at high pressure are delivered to the individual moulding stations by means of a distribution system comprising tubing and a rotating manifold. At each moulding station, near the seal, the presence of a valve assembly is essential in order to:
establish selective communication between the pathway of the air under medium pressure and the parison placed in the closed mould;
establish selective communication between the pathway of the air under high pressure and the parison placed in the closed mould;
enable air to be discharged from the moulded container externally of the mould, passing through a damper.
However, the apparatus and systems described thus far pose a number of problems in the case of use under aseptic conditions.
As is well known, aseptic moulding takes place in an environment that is washable, sterilizable and kept under microbiological contamination control. It is thus fundamental to ensure a correct filtering of the gaseous fluids to be introduced into the controlled environment, correct management of the pressures in the different areas so as to control the path of any unwanted particles, correct monitoring of the environment, and a correct management and adequate quality of the C.I.P. (acronym of “Cleaning In Place”) cleaning and S.I.P. (acronym of “Sterilization In Place”) sterilization cycles.
To this end, the Applicant has recently developed an aseptic moulding apparatus in which the rotating moulding carousel is protected by an isolator suitable for defining a contamination-controlled environment, whereas the driving means of the carousel and mould are located outside said isolator. For the purpose of adapting to aseptic technology, “ad hoc” solutions have been conceived for the seal and the stretching rod (European patent application no. 10425259, PCT patent application published as WO2011/030183).
However, the problem of obtaining a complete sterilization of the circuit supplying air to be blown into the parison has remained unsolved to date. In fact, while the HEPA filters, tubing and rotating manifold are easily sterilizable, that is not the case with the valve assembly.
In order to prevent the sterilized moving parts of the valves from coming periodically into contact with non-sterilized surfaces, thereby becoming vehicles of contamination, valves have already been developed which are provided with a membrane capable of isolating the supply circuit from the drive circuit (see, for example, document WO2011/042184).
However, there still remains the problem of isolating the line for exhausting air from the parison towards the outside environment. During blowing, this exhaust line would be exposed to contamination originating from the outside environment, and should thus remain closed.
On the other hand, safety and accident prevention regulations require that a compressed air circuit be completely emptied and maintained empty both during maintenance operations and during emergency shutdowns. Therefore, the discharge valve should be normally open. In this context, the technical task at the basis of the present invention is to propose a circuit for exhausting air from a parison, a system for supplying and exhausting air from a parison and an aseptic moulding process using said circuit and said system, which overcome the above-mentioned drawbacks of the prior art.